U.S. patent application number 13/381434 was filed with the patent office on 2012-06-28 for springs and spring assemblies.
Invention is credited to Paul Brian Bailey, Michael William Dadd.
Application Number | 20120160091 13/381434 |
Document ID | / |
Family ID | 41008743 |
Filed Date | 2012-06-28 |
United States Patent
Application |
20120160091 |
Kind Code |
A1 |
Dadd; Michael William ; et
al. |
June 28, 2012 |
SPRINGS AND SPRING ASSEMBLIES
Abstract
Various arrangements are disclosed based on springs formed from
a plurality of individual resilient arms, including a spring having
a substantially planar form in an unloaded state and rotational
symmetry of at least order two about a symmetry axis perpendicular
to the plane of the spring, said spring comprising: an inner hub
and an outer hub, said inner hub being radially inward with respect
to said outer hub; a plurality of resilient connecting arms each
connected at an inner end to said inner hub and at an outer end to
said outer hub, said plurality of resilient connecting arms being
configured to provide a restoring force parallel to said symmetry
axis when said inner and outer hubs are displaced relative to each
other along said symmetry axis; wherein said inner hub, said outer
hub and each of said plurality of resilient connecting arms are
formed as physically separate elements and are connected to each
other during assembly to form said spring.
Inventors: |
Dadd; Michael William;
(Summertown, GB) ; Bailey; Paul Brian;
(Summertown, GB) |
Family ID: |
41008743 |
Appl. No.: |
13/381434 |
Filed: |
June 3, 2010 |
PCT Filed: |
June 3, 2010 |
PCT NO: |
PCT/GB10/01100 |
371 Date: |
March 16, 2012 |
Current U.S.
Class: |
92/132 ; 267/160;
29/225; 29/896.9 |
Current CPC
Class: |
F16F 1/10 20130101; F16F
3/02 20130101; F16F 1/027 20130101; Y10T 29/49609 20150115; Y10T
29/53613 20150115 |
Class at
Publication: |
92/132 ; 29/225;
29/896.9; 267/160 |
International
Class: |
F01B 31/00 20060101
F01B031/00; B21F 35/00 20060101 B21F035/00; F16F 3/02 20060101
F16F003/02; B23P 19/04 20060101 B23P019/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 3, 2009 |
GB |
0911629.4 |
Claims
1. A spring having a substantially planar form in an unloaded state
and rotational symmetry of at least order two about a symmetry axis
perpendicular to the plane of the spring, said spring comprising:
an inner hub and an outer hub, said inner hub being radially inward
with respect to said outer hub; a plurality of resilient connecting
arms, each connected via an inner planar attachment region at an
inner end to said inner hub and via an outer planar attachment
region at an outer end to said outer hub, the planes of said inner
and outer planar attachment regions being perpendicular to said
symmetry axis, said plurality of resilient connecting arms being
configured to provide a restoring force parallel to said symmetry
axis when said inner and outer hubs are displaced relative to each
other along said symmetry axis; wherein said inner hub, said outer
hub and each of said plurality of resilient connecting arms are
formed as physically separate elements and are connected to each
other during assembly to form said spring.
2. A spring according to claim 1, further comprising hub location
features for aligning said resilient connecting arms relative to
said inner and outer hubs.
3. A spring according to claim 2, wherein said hub location
features comprise cylindrical holes in said resilient connecting
arms and inner and outer hubs for cooperation with cylindrical
alignment members insertable within said cylindrical holes.
4. A spring according to claim 1, wherein said resilient connecting
arms are arranged in a plurality of stacks, each resilient
connecting arm in a given stack being aligned with all of the other
resilient connecting arms in the same stack along a direction
parallel to said symmetry axis.
5.-8. (canceled)
9. A spring according to claim 1, wherein said hub location
features comprise a fully-constraining location feature and a
partially-constraining location feature, wherein: said
fully-constraining location feature is such as to prevent any
lateral movement within said plane of one of said inner or outer
ends of said resilient connecting arms relative to a portion of the
hub to which said end is connected; and said partially-constraining
location feature is such as to prevent movement in a first
direction within said plane of one of said inner or outer ends of
said resilient connecting arms relative to a portion of the hub to
which said end is connected but allows movement in a second
direction within said plane, said second direction being
perpendicular to said first direction.
10. A spring according to claim 9, wherein said hub location
features comprise a plurality of said fully-constraining location
features and a plurality of said partially-constraining location
features, the numbers and orientations of each being arranged such
that said spring is not over-constrained.
11. (canceled)
12. A spring according to claim 1, further comprising fastening
means for fastening said plurality of resilient connecting arms to
said inner and outer hubs, said fastening means being configured to
apply a fastening force that presses said inner and outer planar
attachment regions against the inner and outer hubs in a direction
parallel to said symmetry axis.
13. A spring according to claim 12, wherein said fastening means
comprise screws engaging with threaded holes in said inner and
outer hubs via corresponding holes in said resilient connecting
arms, and wherein said fastening means are configured to perform
both of: 1) fastening said resilient connecting arms to said inner
and outer hubs; and 2) aligning said resilient connecting arms with
said inner and outer hubs and/or aligning said resilient connecting
arms with each other within a given stack.
14. (canceled)
15. A spring according to claim 13, wherein said screws comprise an
elongated shoulder portion having a plain cylindrical outer surface
engaging against the cylindrical inner surface of holes in said
resilient connecting arms so as to align the resilient connecting
arms within each stack with each other.
16. A spring according to claim 15, wherein said threaded holes
comprise a non-threaded portion having a cylindrical inner surface
and a threaded portion, said elongated shoulder portions engaging
against the cylindrical inner surface of said non-threaded portions
of said threaded holes so as to align said resilient connecting
arms with said inner and outer hubs.
17. A spring according to claim 13, further comprising cylindrical
tubes around the outside of the threaded portions of said screws,
the outer surface of said cylindrical tubes engaging against the
cylindrical inner surface of holes in said resilient connecting
arms so as to align resilient connecting arms in each stack with
each other.
18. A spring according to claim 17, wherein said threaded holes
comprise a non-threaded portion having a cylindrical inner surface
and a threaded portion, said cylindrical tubes engaging against the
cylindrical inner surface of said non-threaded portions of said
threaded holes so as to align said resilient connecting arms with
said inner and outer hubs.
19. A spring according to claim 12, wherein said fastening means
are configured to fix each end of said resilient connecting arms to
said inner and outer hubs via at least two fixing points.
20. A spring according to claim 1, wherein each of said resilient
connecting arms is tapered so as to be thinner in a region towards
the middle of said arm relative to regions nearer said inner and
outer ends of said arm.
21. (canceled)
22. A spring according to claim 4, wherein each of the resilient
connecting arms in each stack is separated from other resilient
connecting arms in the same stack by shims.
23. A spring according to claim 1, wherein the number of stacks of
said resilient connecting arms, or the number of said resilient
connecting arms if there are no stacks, is greater than the order
of said rotational symmetry, and wherein said outer hub comprises a
plurality of disconnected segments, each segment being attached
only to outer ends of resilient connecting members that are nearest
neighbours to each other.
24.-26. (canceled)
27. A spring assembly comprising: a plurality of springs according
to claim 1, axially aligned, disposed as separated groups and
connected together.
28. A kit for constructing a spring according to claim 1,
comprising: an inner hub and an outer hub, physically separate from
each other; a plurality of resilient connecting arms, physically
separate from each other and from said inner and outer hubs and
connectable at an inner end to said inner hub and at an outer end
to said outer hub, said plurality of resilient connecting arms
being configured when attached to provide a restoring force
parallel to said symmetry axis when said inner and outer hubs are
displaced relative to each other along said symmetry axis.
29. (canceled)
30. A piston/cylinder assembly comprising: a spring according to
claim 1; a cylinder connected to one of said inner and outer hubs
of said spring; a piston connected to the other one of said inner
and outer hubs of said spring and configured to move longitudinally
within said cylinder; wherein: said spring is arranged to guide
relative motion of said piston within said cylinder such that a
constant clearance is maintained between said piston and said
cylinder.
31. A method of manufacturing a spring having a substantially
planar form in an unloaded state and rotational symmetry of at
least order two about a symmetry axis perpendicular to the plane of
the spring, said method comprising: providing as physically
separate elements an inner hub, an outer hub and a plurality of
resilient connecting arms; arranging said inner hub so as to be
radially inward with respect to said outer hub; connecting each of
said plurality of resilient connecting arms via an inner planar
attachment region at an inner end to said inner hub and via an
outer planar attachment region at an outer end to said outer hub,
the planes of said inner and outer planar attachment regions being
perpendicular to said symmetry axis, such that said plurality of
resilient connecting arms provide a restoring force parallel to
said symmetry axis when said inner and outer hubs are displaced
relative to each other along said symmetry axis.
Description
[0001] The efficiency and lifetime of most present day
thermodynamic machines is achieved through the use of lubricants.
These are materials that are able to transmit large forces between
moving components with low friction and little wear.
[0002] There are a number of applications where the use of
conventional lubricants such as oil can cause problems. For example
in a Stirling cycle cryo-cooler there are piston/cylinder
assemblies. If these are driven by conventional crank mechanisms
then the use of lubricants is highly desirably for long life and
good efficiency. However the cooler system also includes heat
exchangers that may be operating at a low temperature e.g. 100 K or
below. If oil is allowed to migrate from the piston/cylinder area
to the low temperature heat exchangers, the oil will tend to
solidify and block up the heat exchanger passages with resulting
loss of performance.
[0003] A technology that has been developed to avoid this problem
dispenses with crank mechanisms altogether and instead adopts an
entirely linear configuration. The large side loads associated with
crank mechanisms are avoided and oil free operation is possible.
Typical components of such a system are: a linear suspension system
that uses flexures to define an accurate linear motion; a
piston/cylinder assembly where the piston is guided by the
suspension system so as to maintain a constant clearance within the
cylinder; and a linear transducer, attached to the moving
components, which allows power to be transferred into and out of
the moving assembly.
[0004] This technology has been successfully used in cooling
systems for space applications and has become of increasing
interest in other applications where oil-free operation is
desirable.
[0005] A typical linear suspension system consists of two sets of
flexures. These are components that are able to provide relative
movement by flexing i.e. by elastic deformation. Flexing removes
the need for movement between contacting surfaces and eliminates
the possibility of friction or wear. Flexures are generally made
from thin metal sheet such as stainless steel.
[0006] A design of flexure that has been widely used is commonly
referred to as a spiral spring. Two examples 100 and 300 are
illustrated in FIGS. 1 to 4.
[0007] Spiral springs are formed from a thin, flat disc of
material. The disc is machined so as to produce two or more
rotationally symmetric curved flexing arms 101/301 that connect
inner attachment areas 102/302 to outer attachment areas 104/304 at
the periphery.
[0008] In FIGS. 1 and 2, the inner and outer attachment areas
(clamp areas), 102 and 104 respectively, are in the form of annuli.
FIG. 1 shows the profile of the spring 100. FIG. 2 shows the
clamped areas 102/104 as hatched regions. FIGS. 3 and 4 show an
alternative arrangement 300, where the inner and outer attachment
areas, 302 and 304 respectively, are more complex in shape. In this
instance, the outer clamping area 304 is a number of individual
pads. FIG. 3 shows the profile of the spring 300 and FIG. 4 shows
the clamping areas 302/304 as hatched regions. In both designs, the
spring 100/300 is of one piece construction.
[0009] Axial movement between the inner 102/302 and outer 104/304
attachment areas causes the resilient connecting arms 101/301 to
flex and providing that the material does not yield this movement
is both reversible and repeatable.
[0010] A single spring has poor torsional stiffness about an axis
within the plane of the spring. However if two or more springs are
aligned and connected to form two axially separated sets then the
torsional stiffness is greatly increased. A complete assembly can
then operate as a type of linear bearing where the movement is
entirely taken up by the flexing of the resilient connecting arms
and wear/friction processes are eliminated. The resilient
connecting arms are shaped to minimise the maximum stress for a
given deflection. Providing the stress is not high enough to
fatigue the material, the spring can have a very long life--in
principle if it is not subject to fretting or corrosion, the life
can be infinite.
[0011] An example of such an assembly 500 is shown in FIG. 5. In
this example, there are two sets of two springs 508A/508B/510A/510B
connected by inner and outer hubs. Within each set of springs
508/510, the individual springs 508A/508B and 510A/510B are
separated by shims 512. These are used to prevent contact between
neighbouring springs as such contact can cause fretting and
premature failure. The axial stiffness of the complete assembly 500
can be increased by increasing the number of springs but it is
generally desirable to keep the number down by making individual
springs as stiff possible.
[0012] The linear motion defined by a spiral spring suspension
system is a consequence of the rotational symmetry of the spring's
geometry. The accuracy of the linear movement is therefore limited
by the accuracy of the geometry that is actually achieved. This in
turn is determined by both the manufacture of the spring and the
accuracy with which the spring is clamped. The positions of
clamping forces set the boundary conditions for the resilient
connecting arm deflection and these also need to have accurate
rotational symmetry.
[0013] In existing spring designs the accuracy of the spring
geometry is ensured by making the spring a single component. The
resilient connecting arm profiles are then all machined at the same
time and the accuracy is limited primarily by the machining
process. A machining process that has been widely used is a
chemical process referred to as photo-etching.
[0014] The accuracy with which the clamping forces are positioned
has generally been set by standard engineering tolerances used with
the fasteners. For example, if the springs are clamped with M3
screws then the clearance holes are typically 3.2 mm in diameter,
giving a location error of +/-0.1 mm. While this level of precision
is not very high, experience has shown that it is good enough for
many applications.
[0015] For a flexure bearing in a compressor application, there is
typically a requirement to achieve (or exceed) minimum values of
stroke, radial stiffness and axial stiffness for a specified
envelope and operating frequency.
[0016] The radial stiffness generally needs to be substantially
higher than the axial stiffness and this is achieved by arranging
for the minimum resilient connecting arm width to be several times
bigger than the spring thickness. For example for a width of 2 mm
and spring thickness of 0.25 mm the aspect ratio is 8. The ratio of
stiffness is roughly proportional to the cube of the aspect ratio
and this will result in a stiffness ratio of .about.500.
[0017] The total number of flexure cycles in the lifetime of the
compressor is very high (typically >10.sup.9), so the flexure is
clearly subject to fatigue and this must be considered in the
design.
[0018] The stroke and stiffness requirements generally make
conflicting demands. For example, reducing the thickness of a
spring will reduce the stress in the resilient connecting arms for
a given deflection and will allow a larger stroke for a given
maximum stress level. However, reducing the thickness will also
reduce the axial and radial stiffness values. One of the main
challenges in designing spiral springs is to achieve high values of
both stiffness and stroke for a given envelope (i.e. size/shape of
spring cross-section, perpendicular to axis of displacement). In
general, this translates to using a material that has a high
fatigue strength in a design that has an even stress
distribution--in particular it is important to avoid local stress
concentrations.
[0019] Finite element stress analysis software has been
successfully used to improve spiral spring designs. Whilst it is
not usually possible to arrive at a completely uniform stress
distribution, local peak values can be reduced, resulting in
significantly better performance.
[0020] The stress at which a component is expected to fail is not
single valued, but is determined by the previous stress history, as
failure may be due to an accumulation of damage. A particular
instance of importance in engineering is where the component stress
is varied over many cycles. This type of stress regime is termed
fatigue and it is generally found that the stress at which failure
occurs decreases with an increasing number of cycles. Different
materials demonstrate differing behaviour when subjected to cyclic
stress. For some the strength continues to decline as the number of
cycles increases. For others, it is found that the strength
decreases to a certain value then remains constant. Materials that
exhibit the latter behaviour are said to have a fatigue limit and
the associated minimum strength is referred to as the fatigue
strength. In applications where fatigue is significant, materials
that exhibit a fatigue limit are preferred. Ferrous alloys
generally demonstrate a high fatigue limit and these are the most
generally used. For some materials there is a correlation between
fatigue strength and the hardness (and yield strength) of the
material; materials with high fatigue strength are often very
hard.
[0021] Special grades of stainless steel have been developed for
compressor valve applications where fatigue resistance is a major
requirement. These alloys have very high fatigue strength, good
corrosion resistance and are available in flat sheet form. They are
not expensive, they are very well suited to spiral spring
applications and have been used widely used. An example of such an
alloy is Sandvik's "Hiflex".
[0022] Achieving high fatigue strengths in a finished component can
be difficult. Choosing the material is clearly important as it
determines the maximum fatigue strength that can be achieved.
Equally important is consideration of the manufacturing process as
this can have a big influence.
[0023] The nominal fatigue strength of a material is a value that
sets a maximum peak stress value that can occur anywhere in
component subject to fatigue. Features of a component's geometry or
surface finish can give rise to localised peak values that are
significantly higher than the mean stresses that might be
calculated. Such features, termed stress raisers, are very
detrimental and can lead to failure at low mean stress values. For
example, cracks in a surface that is subject to a cyclic tensile
stress may cause premature failure well below fatigue values based
on mean stresses.
[0024] Machining processes generally degrade the fatigue properties
of a material by leaving cracks in the surface or by locally
heating the alloy and changing its temper. Processes such as
"super-finishing" have been developed that can remove surface
damage after machining, resulting in much improved fatigue
performance.
[0025] In the manufacture of the spiral spring it is desirable to
find a combination of machining and finishing processes that allow
the full fatigue strength of the material to be utilised. In
existing designs the spiral arms are usually defined by thin slots
in the material, and typically the regions where peak stresses
occur are along the sides and ends of these slots. The one-piece
design does not allow the effective use of finishing processes
because the edges along the spiral arms that are highly stressed
are inaccessible to the media that are used in these processes.
This restricts the choice of machine processes to those that leave
a usable surface finish without such finishing processes.
[0026] Photo-etching of spring profiles has been a machining
process that has been widely used. It can result in high quality
machined components that show reasonably good fatigue resistance,
but it has a very significant disadvantage in that quality control
is not very easy, yields can be low, and the chemical process
involved may result in the formation of microcracks on the etched
surfaces.
[0027] The manufacturing cost for existing spring designs is
relatively high because photo-etching is not a low cost
manufacturing process and this basic cost is increased
significantly when quality control and low yields are taken into
account.
[0028] In summary, in the prior art described above, the attainment
of an accurate spring geometry is achieved by machining the spring
as a single component. While this approach is perceived as being
advantageous from the point of view of producing the required
accuracy, it significantly restricts the machining and finishing
processes that can be used. Ultimately, this results in springs
that are relatively expensive and which do not make the most of the
material properties. The present invention seeks to address at
least some of these shortcomings of the prior art.
[0029] According to an aspect of the invention, there is provided a
spring having a substantially planar form in an unloaded state and
rotational symmetry of at least order two about a symmetry axis
perpendicular to the plane of the spring, said spring comprising:
an inner hub and an outer hub, said inner hub being radially inward
with respect to said outer hub; a plurality of resilient connecting
arms, each connected via an inner planar attachment region at an
inner end to said inner hub and via an outer planar attachment
region at an outer end to said outer hub, the planes of said inner
and outer planar attachment regions being perpendicular to said
symmetry axis, said plurality of resilient connecting arms being
configured to provide a restoring force parallel to said symmetry
axis when said inner and outer hubs are displaced relative to each
other along said symmetry axis; wherein said inner hub, said outer
hub and each of said plurality of resilient connecting arms are
formed as physically separate elements and are connected to each
other during assembly to form said spring.
[0030] Thus, in contrast to single piece spiral springs where the
inner and outer hubs are integral with each other, the inner and
outer hubs according to this disclosure are connected to each other
via separate, resilient connecting arms. The spring is assembled by
aligning the inner and outer hubs and the resilient connecting arms
and fastening them together via the inner and outer planar
attachment regions. This process can be facilitated by location
features provided in the hubs and connecting arms.
[0031] The hubs and connecting arms are physically separate in the
sense that they are manufactured as individual items and each has
an identifiable external surface that defines its extent. When they
are brought into contact with each other, there will generally be
an identifiable interface between them (marking a discontinuity in
composition, structure and/or lattice orientation, for example).
There may be a degree of mixing between the materials of two
elements as they are connected together. For example, spot welding,
soldering and/or certain adhesives may cause such mixing, but there
will still be an interface marking the point at which one element
ends and the next element begins. In the case of spot welding, some
melting and mixing of both interfaces might occur, but this is
localized and does not destroy the interface between the elements,
nor otherwise make it impossible to distinguish one element from
another (i.e. to determine that the two elements have been
connected together rather than formed as a single integral
component). In the case of soldering and adhesives, the presence of
these different materials will itself help to define the interface
separating the elements. In prior art arrangements where the hubs
and connecting arms are formed from a single piece of material, by
contrast, no such interfaces are present and the hubs and
connecting arms are integral with each other rather than being
separate items.
[0032] The accurate geometry required for the spring may be
achieved by the use of location features present in the hubs and
the connecting arms. Such location features may be intrinsic to the
spring arm function of the hubs and the connecting arms (i.e. they
contribute to the restoring force provided by the spring).
Alternatively or additionally, they may be added purely for the
purposes of alignment.
[0033] In some instances the fastening means may not be readily
removed, e.g. where the means involves soldering, welding or
adhesive processes. In these instances, the assembly may only occur
once and the components cannot be disassembled for reuse.
[0034] Where the fastening means can be readily removed (e.g. where
bolts or rivets are used), the components will detach from each
other automatically or be detachable with a minimum of applied
force and without any damage to the components. After the
components have been detached in this way, they may be reassembled.
The assembly process may thus be reversible.
[0035] In general, the fastening means will act to press the inner
and outer planar attachment regions against the inner and outer
hubs in a direction parallel to the symmetry axis of the
spring.
[0036] The difficulties associated with manufacturing the single
piece spiral springs of the prior art have been discussed above.
These include the fact that a one-piece design does not allow the
effective use of many finishing processes (such as "super
finishing") because the edges along the spiral arms that are highly
stressed are inaccessible to the media that are used in these
processes. This restricts the choice of machine processes to those
that leave a usable surface finish without such finishing processes
being applied. An example of such machine processing is
photo-etching, but this technique has several disadvantages,
including that quality control is difficult, yields can be low and
the chemical process involved can leave micro-cracks in the etched
surfaces.
[0037] The present invention avoids these problems because the
resilient connecting arms are not integral with the inner and outer
hubs and can be treated and finished as separate items (i.e. prior
to assembly of the spring). There are no narrow gaps or slots which
would cause problems for finishing processes. Indeed, the present
invention could be manufactured effectively using a combination of
stamping (or fine blanking) and super-finishing, which could
greatly reduce costs in comparison with the photo-etching based
manufacture. This process is widely used to manufacture low cost
flapper valves (reed valves) for refrigerator compressors where
high fatigue strength is also an important requirement. Stamping is
not a viable manufacturing process for existing one piece spring
designs as the dies required to produce the thin slots in
(typically hard) material would be very expensive and not
durable.
[0038] Location features may be provided for assisting alignment of
the resilient connecting arms with the inner and outer hubs. These
may comprise, for example, cylindrical holes in the resilient
connecting arms and inner and outer hubs for cooperation with
cylindrical alignment members insertable within the cylindrical
holes. This arrangement is simple to assemble and manufacture has
been shown to be capable easily of providing a level of alignment
that is superior to that achieved in existing single piece spiral
springs. The cylindrical alignment members may be left in the holes
after alignment has been achieved to help maintain alignment or
they can be removed.
[0039] The resilient connecting arms may be arranged in a plurality
of stacks, each resilient connecting arm in a given stack being
aligned with all of the other resilient connecting arms in the same
stack, along a direction parallel to the symmetry axis. This
arrangement allows the stiffness of the spring to be adapted
(increased) easily to the demands of a particular application,
without requiring any modification of the basic resilient
connecting arms. A wide variety of springs can therefore be
manufactured (assembled) using identical resilient connecting arms,
thus allowing significant economies of scale to be made in the
manufacturing process.
[0040] Location features can be provided that serve to align the
resilient arms both with each other (within a stack) and with
respect to the inner and outer hubs. For example, the location
features may comprise cylindrical holes and/or slots and
corresponding cylindrical alignment members. An advantage of using
the same location features for the two functions is that it
minimizes the space required for the location features, which may
be important where the overall envelope (cross-sectional area in
the plane of the spring) is to be minimized.
[0041] The location features may comprise a mix of
fully-constraining location features and partially-constraining
location features. The fully-constraining location features will
prevent any movement within the plane of the spring of the part of
the resilient connecting arms that are in contact with the location
features and the partially-constraining location features will
prevent movement within the plane of the spring of the part of the
resilient connecting arms that are in contact with the location
features only in certain predefined directions. For example, the
partially-constraining location features may consist of slots that
allow linear movement along a predefined line only. This
combination of fully-constraining and partially-constraining
location features may be deployed in such a way as to ensure that
the spring is not over-constrained when there are a large number of
resilient connecting arms. For example, this aim may be achieved by
appropriate selection of the number of fully-constraining location
features, of the number of partially-constraining location
features, and of the nature (e.g. orientation) of the
partially-constraining location features. Over-constraining can
unnecessarily and unpredictably stress the spring, which can lead
to reduced performance and/or premature failure.
[0042] The resilient connecting arms may be fastened to the inner
and outer hubs using a variety of different techniques. For
example, they may be connected using screws that cooperate with
threaded holes formed in the inner and outer hubs. Contact between
the screw head and the resilient connecting arms may be made
through a clamping member or washer.
[0043] The fastening means may be configured also to align the
resilient connecting arms with each other (within a stack) and/or
with the inner and outer hubs. For example, the fastening means may
comprise a screw with an elongated shoulder portion that cooperates
with the interior cylindrical surface of holes formed in the
resilient connecting arms and/or with a non-threaded portion of a
cylindrical hole formed in the inner or outer hubs.
[0044] Combining the fastening means with the hub and/or stack
location features reduces the space necessary for these features,
which may be useful when the spring is to be miniaturized, for
example. The absence of separate location features also means that
there is more space available for the fastening means. Thicker
screws than would otherwise be possible can therefore be used, for
example, which may improve reliability and/or performance.
[0045] Alternatively or additionally, cylindrical tubes may be
provided for insertion around part of the threaded portion of
standard screws. The cylindrical tube engages with the interior
cylindrical surface of holes formed in the resilient connecting
arms and/or with a non-threaded portion of a cylindrical hole
formed in the inner or outer hubs in order to carry out the
alignment function, while a part of the screw thread protruding
from the end of the cylindrical tube engages with a corresponding
threaded portion within the inner or outer hub to carry out the
fastening function. An advantage of this approach is that it allows
standard screws to be used, which are very cheap, and the
cylindrical tubes can be manufactured more cheaply than screws with
elongated shoulder portions (where the profile and axial
orientation of the shoulder portion is to be formed sufficiently
accurately).
[0046] The fastening means may be configured to fix each end of
said resilient connecting arms to said inner and outer hubs via at
least two fixing points. This arrangement has been found most
efficient for supporting the rotational forces arising on axial
displacement (loading) of the spring.
[0047] Each of the resilient connecting arms may be tapered (i.e.
"waisted") so as to be thinner in a region towards the middle of
the arm relative to regions nearer the inner and outer ends of the
arm. For example, the resilient connecting arms may have roughly
the same width at the two ends of the connecting arm and get
continuously thinner towards the middle of the connecting arm. This
arrangement may help to allow a small amount of rotation of the
inner hub relative to the outer hub on axial deflection (loading)
of the spring, with a minimum of additional stress on the resilient
connecting arms. The arrangement also generally helps to reduce
peak stress in the arms and allows efficient nesting of the
arms.
[0048] The number of stacks of resilient connecting arms, or the
number of resilient connecting arms if there are no stacks, may be
arranged to be greater than the order of the rotational symmetry.
In other words, the resilient connecting arms may be arranged in a
pattern that has a lower order of rotational symmetry than the
maximum which is possible for the number of resilient connecting
arms provided. This provides greater flexibility for controlling
the shape of the spring envelope (i.e. its profile when viewed
along the symmetry axis). For example, the profile may be arranged
so as to be relatively compact in a first direction within the
plane and relatively elongated in a second direction with the
plane, perpendicular to the first direction. This might allow the
spring to be used in a device or location with similar spatial
constraints.
[0049] Furthermore, the outer hub may be configured so as to
consist of a plurality of connected or disconnected segments. Where
the segments are disconnected, each segment is attached only to
outer ends of resilient connecting arms that are nearest neighbours
to each other. For example, where the resilient connecting arms are
arranged so that the spring profile is compact in a first direction
and elongated in a second, perpendicular direction, the segments
may be arranged so as to lie substantially parallel to the first
direction. Again, arranging the outer hub in this way provides
greater flexibility for controlling the shape of the spring
envelope.
[0050] It is also noted that where the segments are disconnected,
additional location features will be required to define the spring
geometry. This can be achieved by using more fully the alignment
features that are already present in the resilient connecting arms.
Alternatively, it is possible to use a fixture to connect the
segments during the assembly process.
[0051] According to an alternative aspect of the invention, a
spring assembly may be formed using a plurality of springs
according to the invention connected together in series. Such an
assembly can easily be arranged to have much greater stiffness
against non-axial components of deflection than a single
spring.
[0052] According to a further alternative aspect of the invention,
there is provided a kit for constructing a spring according to
embodiments of the invention, comprising: an inner hub and an outer
hub, physically separate from each other; a plurality of resilient
connecting arms, physically separate from each other and from said
inner and outer hubs and connectable at an inner end to said inner
hub and at an outer end to said outer hub, said plurality of
resilient connecting arms being configured when attached to provide
a restoring force parallel to said symmetry axis when said inner
and outer hubs are displaced relative to each other along said
symmetry axis.
[0053] According to a further alternative aspect of the invention,
there is provided a piston/cylinder assembly comprising: a spring
according to embodiments of the invention; a cylinder connected to
one of said inner and outer hubs of said spring; a piston connected
to the other one of said inner and outer hubs of said spring and
configured to move longitudinally within said cylinder; wherein:
said spring is arranged to guide relative motion of said piston
within said cylinder such that a constant clearance is maintained
between said piston and said cylinder.
[0054] According to a further alternative aspect of the invention,
there is provided a method of manufacturing a spring having a
substantially planar form in an unloaded state and rotational
symmetry of at least order two about a symmetry axis perpendicular
to the plane of the spring, said method comprising: providing as
physically separate elements an inner hub, an outer hub and a
plurality of resilient connecting arms; arranging said inner hub so
as to be radially inward with respect to said outer hub; connecting
each of said plurality of resilient connecting arms at an inner end
to said inner hub and at an outer end to said outer hub such that
said plurality of resilient connecting arms provide a restoring
force parallel to said symmetry axis when said inner and outer hubs
are displaced relative to each other along said symmetry axis.
[0055] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying drawings in
which corresponding reference symbols indicate corresponding parts,
and in which:
[0056] FIG. 1 depicts a single piece spiral spring according to the
prior art;
[0057] FIG. 2 shows the locations of clamping regions in the inner
and outer regions of the spiral spring of FIG. 1;
[0058] FIG. 3 depicts an alternative single piece spiral
spring;
[0059] FIG. 4 shows the locations of clamping regions in the inner
and outer regions of the spiral spring of FIG. 3;
[0060] FIG. 5 shows a linear bearing assembly using axially
separated sets of spiral springs;
[0061] FIG. 6 shows a spring according to an embodiment of the
invention;
[0062] FIG. 7 shows the inner and outer hubs of the spring of FIG.
6 without the attached resilient connecting arms;
[0063] FIG. 8 shows an example of a resilient connecting arm and a
corresponding profile for shims and clamping plates;
[0064] FIG. 9 shows a resilient connecting arm stack comprising two
resilient connecting arms;
[0065] FIG. 10 shows how a resilient connecting arm stack may be
connected and aligned with an inner or outer hub;
[0066] FIG. 11 shows a spring according to an alternative
embodiment, with a reduced number of alignment holes;
[0067] FIG. 12 shows the inner and outer hubs of the spring of FIG.
11 without the attached resilient connecting arms;
[0068] FIG. 13 illustrates the use of shoulder screws to carry out
fastening and aligning of the resilient connecting arms with
themselves and/or with the inner or outer hubs;
[0069] FIG. 14 illustrates the use of cylindrical tubes in
combination with threaded fasteners for carrying out fastening and
aligning of the resilient connecting arms with themselves and/or
with the inner or outer hubs;
[0070] FIG. 15 depicts the inner and outer hubs in an embodiment
using a combination of fully-constraining location features and
partially-constraining location features in the form of slots;
[0071] FIG. 16 depicts a spring where the order of rotational
symmetry is less than the number of resilient alignment stacks and
the spring envelope is elongated; and
[0072] FIG. 17 depicts a sectional view of a piston/cylinder
assembly using springs according to embodiments of the
invention.
[0073] FIG. 6 shows a plan view of a spring 600 according to an
embodiment of the invention, consisting of a number of separate
resilient connecting arms 614 that are attached at inner and outer
planar attachment regions to inner and outer hubs, 616 and 618
respectively, via shims and clamps (to be positioned in
cross-hatched "clamping areas" 621). The planes of the inner and
outer planar attachment regions are generally perpendicular to the
longitudinal, load bearing, symmetry axis of the spring. FIG. 7
shows the profiles of the inner and outer hubs 616/618 with the
resilient connecting arms 614, shims and clamps removed.
[0074] The individual resilient connecting arms 614 with their
inner and outer planar attachment regions 825/827; and profiles 620
for the shims and clamping plates are shown in FIG. 8.
[0075] In general, the resilient connecting arms 614 are not
restricted to having a particular profile but in the embodiment
shown the resilient connecting arms 614 are curved with profiles
that taper towards the middle (i.e. they are "waisted"). The
curvature allows a degree of rotation to occur between the inner
and outer hubs 616/618 during flexing and helps reduce the stress
in the resilient connecting arms 614. The waisted profile likewise
helps reduce the peak stress. In this example, the waisted profile
allows the six resilient connecting arms 614 to nest together to
provide a flexing connection similar to that provided by a single
one piece spiral spring (see FIGS. 1 to 4 for example).
[0076] Similarly, the shape 620 of the shims and clamping plates
and that of the clamping areas 621 on the inner and outer hubs
616/618, and the inner and outer planar attachment regions 825/827
(which may optionally match the shape 620 of the shims and clamping
plates) are not in general restricted to a particular profile. The
clamping area 621, inner and outer attachment regions 825/827, and
shims and clamping plates should generally be sufficiently large,
however, to contain the appropriate fastening and locating features
desired. The shape is generally designed to suit the profile of the
resilient connecting arm, but this need not be the case.
[0077] In the example shown in FIGS. 7 and 8, two larger holes 623A
are provided in both ends of each resilient connecting arm 614,
along with corresponding holes 623B in each of the corresponding
clamping areas 621 on the inner and outer hubs 616/618, to allow
access for fastening means. The fastening means may, for example,
comprise screws that pass through the holes 623A in the connecting
arms 614 and engage with threads in the corresponding holes 623B in
the inner and outer hubs 616/618. Tightening of the screws causes
the inner and outer attachment regions 825/827 of the resilient
connecting arm 614 to be pressed in a longitudinal direction (i.e.
parallel to the symmetry axis of the spring and perpendicular to
the plane of the inner and outer attachment regions 825/827)
against the inner and outer hubs 616/618.
[0078] Two smaller holes 622A are provided in both ends of the
resilient connecting arms 614 to act as part of an alignment
mechanism. A corresponding hole 622B is provided in each of the
clamping areas 621 on the inner and outer hubs 616/618. Optionally,
larger holes could be used for the alignment mechanism and smaller
holes to accommodate fastening means, or the holes for fastening
and alignment could be the same size, or even shared (see
below).
[0079] In general, the alignment mechanism needs to comprise two
elements: features that align the resilient connecting arms 614
with respect to the inner and outer hubs 616/618, which may be
referred to as "hub location features", and features that align the
resilient connecting arms 614 with each other to form "stacks" of
arms 614 parallel to the symmetry axis of the spring 600, which may
be referred to as "stack location features". The hub and stack
location features can provide both positional and angular
alignment. The stack location features may also align shims
inserted between the resilient connecting arms 614 in a given
stack, as well as any clamping plates that are positioned at the
axial extremities of the stacks.
[0080] In preferred embodiments, either or both of the hub and
stack location features may comprise cylindrical holes 623 for
cooperation with cylindrical alignment members (e.g. dowels) that
are a close fit with the cylindrical holes.
[0081] In the example shown in FIGS. 6 and 7, both of the smaller
holes 622A are effective for aligning the resilient connecting arms
614 (and any shims or clamping plates) with each other within the
stack while only one of the two smaller holes 622A need be used for
aligning the resilient connecting arms 614 with the inner and outer
hubs 616/618. This is why the clamping areas 621 contain only a
single smaller hole 622B, for cooperation with the one of the two
smaller holes 622A that is used for hub alignment. Cylindrical
alignment members may be inserted into both of the smaller holes
622A at each end of the connecting arms 614, but only one of the
two cylindrical alignment members will penetrate into a hole 622B
in the inner or outer hub 616/618.
[0082] This arrangement can be seen in more detail in FIGS. 9 and
10.
[0083] FIG. 9 shows a side view of how the resilient connecting
arms 614, clamping plates 924 and shims 926 can be stacked together
to form resilient connecting arm stacks or sub-assemblies 928. In
the example shown, the stack 928 comprises two resilient connecting
arms 614, but more could be included if desired. Two cylindrical
alignment members 930A/B at each end of the resilient connecting
arm 614 are used to achieve stack alignment.
[0084] The orientation of the inner and outer planar attachment
regions 825/827 perpendicular to the longitudinal symmetry axis of
the spring (which is directed towards the right of the page in FIG.
9) can clearly be seen.
[0085] FIG. 10 shows how the ends of each resilient connecting arm
stack 928 may be connected to the inner or outer hub 616/618 via
the inner and outer planar attachment regions 825/827. In this
instance, the location on the hub 616/618 is provided by one of the
cylindrical alignment members 930A penetrating through into the
corresponding hole 622B in the hub 616/618. Thus, the cylindrical
alignment members 930A and corresponding holes 622A in the
resilient connecting arms 614, shims 926 and clamping plates 924
contribute both to hub alignment and stack alignment.
[0086] Once the resilient connecting arms 614 are secured to the
hubs 616/618 by suitable fasteners it is possible to release either
or both of the hub and stack location features. For example, where
cylindrical alignment members 930 are used, these may be removed.
Alternatively, the hub and stack location features can remain
partially or completely in place (e.g. a subset or all of the
cylindrical alignment members 930A/B may remain inserted), which
may help to prevent loss of alignment during use. Elements such as
the cylindrical alignment members 930A/B may be secured in place,
for example by means of adhesive.
[0087] Although it is possible to use single fasteners for
clamping/attaching each end of the resilient connecting arms 614,
it is found that a better attachment is achieved if two fasteners
are used. This is why two holes 623A/B are provided in each end of
the connecting arms 614 and in each clamping area 621 in the
example shown in FIG. 6. In the operation of the spring 600 (i.e.
on axial displacement of the inner hub 616 relative to the outer
hub 618), there is a small rotation of the inner hub 616 relative
to the outer hub 618 associated with the flexing of the resilient
connecting arms 614. This rotation produces bending moments
parallel to the spring's axis that would tend to twist each
resilient connecting arm 614 with respect to its clamped areas. In
a one-piece spiral spring, the bending moment is adequately
resisted by the solid connection between the inner clamping areas.
In the separate resilient connecting arm design described here,
this solid connection does not exist and the moment has to be
resisted by the clamping mechanism. It is difficult to provide
sufficient resistance to this torque using only a single fastener.
Two fasteners (or more) are more able to provide adequate
resistance and the risk of the connection coming undone can be more
easily reduced.
[0088] A further variation is to use the same holes for alignment
(e.g. for insertion of a dowel) and for fastening (e.g. as
clearance holes for a screw). FIG. 11 shows a plan view of such a
spring 1100. FIG. 12 shows the inner and outer hubs, 1116 and 1118
respectively, with the combined alignment and fastener holes 1132.
All the possible locations in the hubs 1116/1118 are shown, but in
practice only one of each pair of combined location and fastener
holes 1132 would be used to locate the resilient connecting arm
assembly onto a hub 1116/1118. The advantage of reducing the number
of holes required in the limited extent of the clamping area is
that it allows larger fasteners to be used. This is particularly
helpful for small machines where it is preferable to avoid the use
of very small fasteners (e.g. M1.6).
[0089] Two possible approaches for combining the alignment and
fastener holes are shown in FIGS. 13 and 14. FIG. 13 shows the use
of shoulder screws 1334/1336. These are screws with an accurately
sized plain cylindrical surface 1334A/1336A below the screw head
1334B/1336B but before the thread 1334C/1336C starts. The plain
cylindrical section 1334A/1336A is sized to engage in a combined
alignment and fastener hole 1132 and acts as a dowel to provide the
alignment required. The combined alignment and fastener hole 1132
comprises a threaded portion to provide fastening and a
non-threaded portion for engaging with the plain cylindrical
section 1334A/1336A of the shoulder screw 1334/1336A to provide
alignment. A challenge with this approach is constructing the
shoulder screws 1334/1336 in a sufficiently precise manner. Not
only does the outside diameter of the plain cylindrical surface
1334A/1336A need to be accurately defined, but also its relation
(accurately coaxial) to the axis of the screw thread 1334C/1336C.
This may be made more difficult by the fact that the choice of
material for the plain cylindrical surface 1334A/1336A is
constrained by the need for the material also to be suitable for
forming the thread. In addition, care needs to be taken to ensure
that deformation of the screw during tightening does not compromise
alignment. Overall, these difficulties mean that shoulder screws of
the required accuracy will be significantly more expensive than
standard fasteners.
[0090] An alternative embodiment that allows standard fasteners to
be used is shown in FIG. 14. The alignment is achieved through the
use of short lengths of cylindrical tubes 1438 that are sized to
give accurate engagement in the combined alignment and fastener
holes 1132 holes. The fasteners 1440 are a clearance fit inside the
alignment tubes 1438, so they have no influence on the
alignments.
[0091] In an alternative embodiment the springs are aligned using
features other than holes and dowels or cylindrical tubes. Examples
of such features are straight edges or notches formed at the ends
of the resilient connecting arms 614 and arranged to cooperate with
corresponding features forming in the clamping areas 621 of the
inner and outer hubs 616/618.
[0092] As mentioned above, alignment of the resilient connecting
arm stacks 928 with respect to the hubs 616/618 can be achieved
through the use of location features that are also used to align
the resilient connecting arms 614 with each other and with the
shims 926 and clamping plates 924. In such an arrangement, if the
inner and outer hubs 616/618 of the spring are connected, for
example, by three resilient connecting arm stacks (or three
resilient connecting arms that are not in stacks), with single
locations at each end, then the spring geometry is fully defined.
It follows from this that if there are only two resilient
connecting arm stacks then an additional alignment feature is
required to define the spring geometry. Where more than one
alignment hole is available at each end of the resilient connecting
arm stacks, the additional location may be implemented using any
one of the unused holes.
[0093] It also follows that if the spring has more than three
resilient connecting arm stacks (or connecting arms not in stacks)
then the use of all the alignment holes could (if each of the
alignment holes is arranged to prevent movement in all directions
within the plane of the spring) over-constrain the system, which
will tend undesirably to stress the spring and reduce its
performance and/or longevity. Alignment features (e.g. an alignment
hole for use with a corresponding dowel) that prevent movement in
all directions with the plane of the spring may be referred to as
"fully-constraining location features".
[0094] FIG. 15 shows the inner and outer hubs, 1516 and 1518
respectively, for a spring 1500 having six resilient connecting arm
stacks 1528. The resilient connecting arms are not shown, but would
be connected in a configuration similar to that of FIG. 11.
Over-constraining of the geometry is avoided by replacing three of
the alignment holes in the clamping areas 1542/1544 with alignment
slots 1540. The slots 1540 give an extra degree of freedom in their
engagement with alignment dowels/tubes. Alignment features (e.g. an
alignment slot for use with a corresponding dowel) that provide a
degree of location but which allow relative movement in at least
one direction within the plane of the spring may be referred to as
"partially-constraining location features". In a preferred
embodiment, the slot 1540 is aligned along the direction connecting
the inner and outer connection points of the resilient connecting
arm, for example along the dotted line 1546 connecting fastener
clearance holes 1548A and 1548B. This ensures that the angular
alignment is accurate and avoids uneven gaps between the individual
resilient connecting arms while not over-constraining the
system.
[0095] The slots 1540 can be placed on the clamping areas 1542 of
the inner or outer hubs provided there is only one slot per
resilient connecting arm. In the example shown in FIG. 15, the
three alignment slots 1540 have been placed evenly around the outer
hub. This approach can be applied to other designs that have
different numbers of arms. It may not be possible to position the
alignment slots evenly but this does not matter. For example, a
spring with eight resilient connecting arms would require five
location slots.
[0096] In general, a minimum of two resilient connecting arms is
required to give the balanced axial deflection necessary for linear
movement. Preferred embodiments will tend to have assemblies in
which the resilient connecting arms are equispaced i.e. if there
are six arms there will be six degrees of symmetry. However, it is
also possible to have resilient connecting arm assemblies that are
not equispaced providing that there are at least two degrees of
rotational symmetry.
[0097] FIG. 16 shows an example of a spring 1600 having only four
resilient connecting arms 614 of a six-arm configuration (such as
that shown in FIG. 6, for example). The arrangement has rotational
symmetry of order two, rather than of order six, which would be the
case if all six resilient connecting arms were used. The inner hub
616 may have a similar construction to the inner hub of the six-arm
configuration. The outer hub 1618A/1618B, on the other hand, can
advantageously be segmented rather than fully annular so as not to
extend unnecessarily in the direction of the missing resilient
connecting arms. More specifically, the outer hub 1618A/1618B may
be constrained to extend (i.e. connect directly) only between the
outer clamping areas that are nearest neighbours with each other,
for example between clamping areas 1644A and 1644B for outer hub
segment 1618A and between clamping areas 1644C and 1644D for outer
hub segment 1618B. The resulting geometry still has two degrees of
rotational symmetry and is capable of defining an accurate linear
movement. One advantage of this arrangement is that it allows a
linear dimension to be reduced and hence enables a more compact
envelope.
[0098] Alternative embodiments can employ means other than threaded
fasteners to clamp the assembly. For instance, in the resilient
connecting arm stacks (such as that in FIG. 9, for example), the
resilient connecting arms, shims and/or clamp plates can be held
together by adhesive, by mechanical means other than those already
mentioned, e.g. riveting, and/or by metallurgical bonding means,
e.g. soldering, brazing or welding. Likewise, the resilient
connecting arm stacks could by secured to the outer and/or inner
hubs by adhesive, by mechanical means other than those already
mentioned, e.g. riveting, and/or by metallurgical bonding means,
e.g. soldering, brazing or welding.
[0099] It will be noted that embodiments of the invention involve
the assembly of a large number of individual components. For
example for the equivalent of 3 springs there may be 138
components. This high component count may seem a disadvantage.
However, there are only six distinct components to be manufactured
and the invention is well suited to automated assembly. For
example, the sub-assembly 928 of FIG. 9 could be easily assembled
robotically as a process that is separate from the main assembly.
There are various means possible for holding the sub-assembly 928
together for easy handling. For example, the alignment tubes could
be bonded to the top and bottom plates. Alternatively, components
can be used to provide sufficient friction on the alignment tubes
to hold the stack together--examples might be small polymer
"olives" between the tube and plate or shims that are slotted and
can act as circlips.
[0100] It will also be noted that the inner and outer hubs may be
individual components that allow a free-standing spring to be
assembled. Alternatively, it is also possible for either or both of
the hubs to be integrated into other components. In the latter
case, the spring assembly cannot be separated from the assembly of
the complete machine and a free-standing spring does not exist as a
separate entity.
[0101] FIG. 17 is a sectional view of a piston/cylinder assembly
using springs according to embodiments of the invention. The two or
more sets of springs are axially aligned, spaced apart and
connected together to provide the required stiffness against
off-axis distortions and ensure accurate linear motion. The springs
are generally connected together such that their forces act in
parallel. The inner hubs 616A/B are connected together via inner
connector 1760 and the outer hubs 618A/B are connected together via
outer connector 1762. The inner connector 1760 and inner hubs
616A/B are fixedly connected to the piston 1750 and the outer
connector 1762 and outer hubs 618A/B are fixedly connected to the
cylinder 1752. Means 1754 are provided for applying an axial force
between the inner and outer connectors 1760/1762 (e.g. using an
electromagnetic linear transducer), which causes a corresponding
axial movement of the piston 1750 within the cylinder 1752, which
is guided by the springs so that a constant clearance is maintained
between the cylinder 1752 and the piston 1750. This arrangement can
be used as the basis for various machines, such as long-life
lubricant free pumps, compressors, Stirling cycle cryo-coolers, or
Stirling engines, for example.
[0102] To help achieve precisely linear axial movement between the
inner and outer hubs 616/618, the resilient connecting arms 614
should, in addition to the hub alignment and stack alignment
discussed above, be aligned with respect to a plane perpendicular
to the axis of linear motion. An example of such alignment might
lead to the following constraints: 1) the surface of the inner hub
616 should be flat (if formed from separate elements, the surfaces
of those elements should be flat and coplanar with each other); 2)
the surface of the outer hub 618 should be flat (if formed from
separate elements, the surfaces of those elements should be flat
and coplanar with each other); and 3) the inner hub 616 should be
coplanar with the outer hub 618 (or the surfaces of the members of
the inner and outer hubs 616/618 should be coplanar).
[0103] An alignment of this nature could be achieved, for example,
by means of an alignment fixture consisting of a plate with one
surface machined flat. The flat face of this fixture could be
fastened to the surfaces of both the inner and the outer hubs
616/618, thereby ensuring that all the surfaces are coplanar. While
the fixture is attached in this fashion, the resilient connecting
arms 614 could be mounted to the inner and outer hubs 616/618 and
fastened in place. The fixture could then be removed.
* * * * *